Iridium is a rare,
hard, lustrous, brittle, very dense platinum-like metal. Chemically
it is very unreactive. It is the most corrosion-resistant metal
known and it resists attack by any acid. Iridium is attacked by
molten salts such as sodium chloride (NaCl) and sodium cyanide (NaCn).
Iridium is generally credited with being the second dense element
(after osmium) based on measured density, although calculations involving the space
lattices of the elements show that iridium is denser.

Atomic
Weight: 192.9

Density: 22.65 g/cm3 at 20 oC

Melting
Point: 2447 oC

Boiling
Point: 4500 oC

Coefficient of Linear Thermal
Expansion: 6.8 µm/m . K at 20oC

Specific
Heat: 0.130 kJ/kg

Thermal
Conductivity: 147 W/m ⋅ K at 0 to 100oC

Electrical
Resistivity: 47.1 nΩ ⋅ m at 0oC; 53 nΩ ⋅ m at 20oC

Temperature
Coefficient: 0.00427 nΩ ⋅ peroC at 0 to 100 oC

Thermal
Electromotive Force: Pt 67 (reference junction at 0 oC):

+3.626
mV at 400 oC

+6.271 mV at 600 oC

+12.741 mV at 1000 oC

Tensile
Properties of Iridium Annealed at 1500 oC:

TemperatureTensile Strength 0.2% Yield Strength
Reduction in Area

oC

MPa

MPa

%

24

623

234

6.8

500

530

234

12.7

750

450

142

51.0

1000

331

43.4

80.6

Magnetic Susceptibility Mass: 0.19 x 10-8 mks at 18 oC

Reflectivity:
64% at λ = 0.45 um

70% at λ = 0.55 um

78% at λ = 0.75 um

Emissivity:
0.30 at 0.65 micron for solid unoxidized metal.

General Corrosion
Behavior:

Iridium is the most corrosion-resistant
element. It is not affected by common acids, including hot sulfuric
acid. It is slightly attacked by sodium hypochlorite solutions but
not by aqua regia at ordinary temperatures. However, at elevated
temperatures and pressures, aqua regia does attack iridium, and it
may be used under these conditions for dissolving iridium and its
refractory alloys for analysis. Iridium is virtually insoluble in
lead even at high temperatures, and use is often made of this fact
in preliminary steps in chemical analysis.

Fabrication Characteristics:

Iridium can be arc melted (inert-gas cover),
electron beam melted, or consolidated by powder metallurgy
techniques. It is hot worked using procedures similar to those used
for tungsten. Final working is done at warm temperatures, which
produce a fibrous structure. Iridium has limited malleability at
room temperature.

Tensile Properties (of 0.5 mm wire):

Tensile
Strength: Annealed at 1000 oC:
1100 to 1240 MPa

Hot Drawn: 2070 to 2480 MPa

Elongation:
Annealed: 20 to 22%

Hot Drawn: 13 to 18%

Hardness:

Annealed at 1000 o: 200 to 240 HV

As-Cast: 210 to 240 HV

Hot Drawn: 600 to 700 HV

Modulus of Elasticity:

Tension:
517 GPa (Static); 527 GPa (Dynamic)

Compression:
210 GPa

Poisson's Ratio: .26

Applications:

The main use of iridium is as a hardening agent for
platinum alloys. With osmium, it forms an alloy that is used for tipping
pens, and compass bearings. Iridium is used in making crucibles and other
equipment that is used at high temperatures. It is also used to make
heavy-duty electrical contacts. Iridium was used in making the
international standard kilogram, which is an alloy of 90% platinum and 10%
iridium. Radioactive isotopes of iridium are used in radiation therapy for
the treatment of cancer.